The cell biology studies have identified a number of HTLV-I isolates from patients with adult T-cell leukemia-lymphoma (ATLL), lymphosarcoma cell leukemia from different parts of the world including Japan, the Caribbean, and southeastern United States. These HTLV-I isolates have been transmitted to T-cells from cord blood cells, peripheral blood and bone marrow. The T-cells infected with HTLV are transformed and have convoluted nuclei, a characteristic similar to the one observed in the patients' tumor cells. HTLV-I has been isolated from family members of a Japanese patient with ATLL. These virus isolates have properties similar to those isolated from the ATLL patient. The HTLV-I infected cells produce a number of lymphokines. The lymphokines that have been identified include MIF (migration inhibitory factor) which inhibits the migration of fresh human macrophages), MAF (macrophage activating factor), DIF (differentiation inducing factor), CSF (colony stimulating factor), EOS-GMA (eosinophil growth and maturation activity), FAF (fibroblast activating factor) and Gamma-interferon. All these lymphokines were detected in unconcentrated tissue culture fluids from most of the HTLV-I positive T cell lines. HTLV-I infected cell lines produce these lymphokines constitutively and are an excellent source for these factors. A variant of HTLV-I was isolated from a patient with hairy cell leukemia (HTLV-II) and more recently from a patient with AIDS. HTLV-II has properties similar to HTLV-I and can infect and transform cord blood cells more efficiently than peripheral blood or bone marrow cells. More recently another HTLV variant (HTLV-III) has been isolated from a number of patients with AIDS and pre-AIDS. HTLV-III is cytopathic like HTLV-I and HTLV-II, but it does not cross-react with monoclonal antibody produced against HTLV-I p19. HTLV-III has been transmitted into a T-cell line which is productively infected. With the availability of this cell line it should be possible to produce large quantities of the virus to study biological and biochemical properties. In addition, studies are in progress to determine if HTLV-I, HTLV-II and HTLV-III can induce leukemia (lymphoma) or AIDS in subhuman primates.